An inactivating mutation within the first extracellular loop of the thyrotropin receptor impedes normal posttranslational maturation of the extracellular domain.
The TSH receptor (TSHR), a member of the large family of G protein-coupled receptors, controls both function and growth of thyroid cells; hence, mutations of this receptor result in thyroid dysfunction. Here, we took advantage of the description of a new inactivating TSHR mutation (Q489H) in two brothers with hypothyroidism, to precise maturation, intracellular trafficking, exporting pathways, and activation mechanisms of this receptor. Functional characterization of the Q489H-TSHR in transiently transfected HEK293 cells showed cell surface expression, normal TSH binding affinity, and its inability to generate intracellular cAMP in response to TSH stimulation. Western blot analysis of the whole membrane proteins or proteins expressed at the cell surface showed that Q489H-TSHR expressed in HEK293 transfected cells are restricted to mannose-rich uncleaved receptor. Analysis of the export pathway toward cell surface indicated that both Q489H and wild-type receptors followed the same intracellular route to cell surface throughout endoplasmic reticulum and Golgi apparatus. This study shows that Q489H substitution impedes complete glycosylation of TSHR extracellular domain within the Golgi apparatus and that Q489H-TSHR expressed at the cell surface is unable to undergo intramolecular cleavage as well as to switch toward an active conformation under TSH stimulation. Altogether, our results show that 1) Q489H substitution within the first extracellular loop induces a misfolding of TSHR, blocking it into an inactive conformation and impeding complete glycosylation and intramolecular cleavage, and 2) a misfolded G protein-coupled receptor can bypass endoplasmic reticulum or Golgi apparatus quality control and reach the cell surface as an immature receptor.
Sura-Trueba
Sylvia
S
Institut National de la Santé et de la Recherche Médicale (INSERM) Unité 690, Hôpital Robert-Debré, Paris, France.
Aumas
Chantal
C
Carre
Aurore
A
Durif
Sylvie
S
Leger
Juliane
J
Polak
Michel
M
de Roux
Nicolas
N
D000328:Adult; D000595:Amino Acid Sequence; D001483:Base Sequence; D002478:Cells, Cultured; D004252:DNA Mutational Analysis; D005190:Family; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D008969:Molecular Sequence Data; D020125:Mutation, Missense / Q000502:physiology*; D010375:Pedigree; D011499:Protein Processing, Post-Translational / Q000235:genetics*; D017434:Protein Structure, Tertiary / Q000235:genetics; D011989:Receptors, Thyrotropin / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
D011989:Receptors, Thyrotropin